1. Field of the Invention
This invention relates generally to cutting or milling tools for cutting materials in wellbores and more particularly to cutting tools utilizing a pressurized fluid for cutting materials in wellbores.
2. Background of the Art
To produce hydrocarbons (oil and gas) from the earth's formations, wellbores are formed to desired depths. The first few hundred feet of the wellbore are typically large in diameter, usually between 12 and 18 inches, and are lined with a metal casing, about one half inch thick or more to prevent caving of the wellbore. The wellbore, which is typically between nine to twelve inches in diameter, is then drilled to recover hydrocarbons from the subsurface formations. After the wellbore has been drilled to the desired depth, a metal pipe, generally referred to in the art as the casing or pipe, is set in the wellbore by injecting cement between the casing and the wellbore annulus. Branch or lateral wellbores are frequently drilled from a main wellbore to form deviated or horizontal wellbores for improving production of hydrocarbons from subsurface formations.
There are many operations (work) to be performed in the wellbore. Often it would be advantageous to be able to "see" (image) a particular worksite, determine what specific work needs to be performed based on the imaging information and then perform the work, preferably with tools that have been run downhole at the same time as the imaging equipment.
The current technique is to run imaging equipment downhole, collect the imaging information and then pull the imaging equipment out of the borehole before running the necessary tool(s) downhole to do the work. The work that may be performed may include: testing, inspection, cutting, fishing, repairing, sealing, welding and/or cementing. Some cutting examples are noted below.
In many applications, the branch wellbores are formed after the wellbore has been cased. This requires milling or cutting a section (window) in the casing at a predetermined depth to initiate the drilling of the branch wellbore. It is highly desirable to cut such windows with enough precision to preserve the eventual junction integrity. In older wellbores, the junctions between the main wellbore and the branch wellbore may be eroded and may require the removal of certain materials therefrom to repair such junctions or to perform secondary operations. It is desirable to remove the materials from the junctions with precision in order to properly reconstruct the junctions. Therefore, it is desirable to have a downhole cutting or milling tool that can selectively and relatively accurately cut windows in the casing downhole and also remove a desired amount of materials around the junctions. The present invention provides such a downhole cutting tool.
After the wellbore has been cased, various types of equipment, such as liner hangers, packers, fluid flow control devices, etc., are installed (set) in the wellbore. Some of these devices are permanently set in the wellbore and must be milled to perform secondary operations. Other devices, although designed to be retrieved, cannot be so removed from the wellbore due to malfunctions of such devices or excessive corrosion and, therefore, these devices must be milled.
Additionally, sediments tend to slowly settle along the interior surfaces of production tubing, which reduces the effective annulus of such tubing. From time to time, such sediments must be reamed to maintain the desired fluid flow through the tubing.
Various types of downhole cutting and milling tools have been utilized in the oil and gas industry. Such tools have been used for removing materials from within wellbores including cutting existing casings, for boring through permanently set packers and for removing loose joints of pipes. Milling tools have been used to ream collapsed casings, to remove burrs or other imperfections from windows in the casings, to place whipstocks for drilling directional wellbores and to perform other reaming operations.
Prior art cutting or milling tools typically include a tool body that is adapted to be conveyed into the wellbore. A plurality of cutting blades are placed on the body at spaced intervals extending outwardly therefrom. Each of the blades typically have a base with a leading surface relative to the direction of rotation. A suitable hard cutting material, such as carbide, is secured to the cutting edge. To perform a cutting or milling operation, the tool is placed at a desired location within the wellbore and rotated to cut the intended material. The weight on the tool and the rotational speed determine the cutting speed. The tool blades are designed to cut the material in small segments so that the cuttings may be transported to the surface by circulating a fluid in the wellbore or dropped to the wellbore bottom. A commonly used downhole cutting tool of the type described above is disclosed in U.S. Pat. No. 4,978,260, issued to Gerald Lynde and assigned to the assignee of this application.
The cutting elements of such prior art must remain in hard contact with the material to be cut, which erodes the cutting elements. The operating life of such cutting elements in some applications, therefore, can be relatively short. In such cases, the cutting tool must be retrieved for changing the cutting element. This type of operation can result in lost time for the well and/or rig. This lost time can cost several thousand dollars per day.
The cutting area of prior art cutting tools is relatively large and, thus, such tools do not cut relatively precise sections or windows in the casings. It is also difficult to orient such prior art cutting tools to perform contoured cutting of areas within the wellbores.
The present invention addresses many of the deficiencies of the prior art cutting or milling tools and provides tools wherein the cutting element is relatively small, does not contact the surface to be cut and can cut materials in a wellbore relatively precisely. The small cutting element enables making precise cuts while the non-contacting aspect of the tool increases the life of the cutting element. The cutting element can be continuously positioned and oriented in the wellbore to perform cutting operations along a predetermined profile or trace, which allows performing relatively precise cutting operations.
Additionally, by incorporating the cutting tool with imaging equipment that is run downhole at the same time as the cutting tool, it is possible to image the worksite, determine the type of cut that needs to be made, sending the proper signals to the cutting tool and perform the cutting operation with a single downhole run.